That's really strange. I doubt anything like this exists but you could certainly make it yourself. All you really need is a step-down voltage regulator that can regulate it down to somewhere in the 2.4-3.0V range (a bit lower or higher might work too), a donor USB cable, a soldering iron, glue and some time. If you want you can mod it into the battery door, they sell replacement battery doors cheaply so you don't have to mod the original one. That would make it possible to switch to batteries at will.